Take a look at a door and the hinges on which it swings. There are probably three or four hinges, each of which consists of three separate parts and four to six screws. Do the math, and you'll see that the hinge you take for granted in your daily comings and goings consists of at least 21 separate components. Life would be so much simpler if the number of parts needed for a hinge could be reduced. The good news is that, at least in the design of plastic parts, it often can.

A reduction to three or two or just one part for a hinge would be notable. A reduction to zero is truly impressive, and that's exactly how many additional parts a living hinge requires. Quite simply, a living hinge is a thin strip molded into a plastic part to create a line along which the part can bend. Properly designed and executed, it can be closed and opened over the life of the part with little or no loss of function. But simple though it may be in concept, a living hinge must follow certain guidelines if it is to work properly.

First, only certain resins are flexible enough to support the degree and frequency of bending required of a hinge. The best resins for parts with living hinges are polyethylene and polypropylene.

When a hinge bends, tensile forces are transmitted to the material along the outside of the bend. The thicker the hinge, the greater the stress in the outside surface, so the hinge should not be too thick or it may crack when it is bent. On the other hand, if the hinge is too thin it will not be strong enough to withstand any tearing forces, especially at the ends. The following geometry (from efunda.com) works well for hinges made of either of the two resins mentioned above.

Fig. 1

Also, be careful that, when the hinge is bent fully, there won't be interference from thick edges along the hinge.

Finally, a thin spot in a part (which is what a hinge is) can be challenging to fill during resin injection. Success depends on proper gate placement. A single gate that forces resin through the hinge area in a mold increases the strength of the hinge; however, this approach can lead to sink in areas downstream from the hinge. On the other hand, multiple gates may eliminate the problem of sink, but if resin flows meet at the hinge (which they will tend to do), they will usually cause cracking. When you order a mold, Protomold will propose gate location(s) to optimize filling of the part including any living hinges.

Fig. 2

If this all seems like a lot of trouble, keep in mind that experts suggest that a well designed living hinge can be flexed millions of times. That's more times than most of us will walk through doors in a lifetime.

Got Cube?

For a hands-on look at living hinges,
see Protomold's folding sample cube. If you haven't already received one, you can get one free. Click on the picture to make your request.